Active Voltage Control in Distribution Networks Including Distributed Energy Resources

Abstract

The structure and control methods of existing distribution networks are planned assuming unidirectional power flows. The amount of generation connected to distribution networks is, however, constantly increasing which changes the operational and planning principles of distribution networks radically. Distributed generation (DG) affects power flows and fault currents in the distribution network and its effect on network operation can be positive or negative depending on the size, type, location and time variation of the generator. In weak distribution networks, voltage rise is usually the factor that limits the network’s hosting capacity for DG. At present, voltage rise is usually mitigated either by increasing the conductor size or by connecting the generator to a dedicated feeder. These passive approaches maintain the current network operational principles but can lead to high DG connection costs. The voltage rise can be mitigated also using active voltage control methods that change the operational principles of the network radically but can, in many cases, lead to significantly smaller total costs of the distribution network than the passive approach. The active voltage control methods can utilize active resources such as DG in their control and also the control principles of existing voltage control equipment such as the main transformer tap changer can be altered. Although active voltage control can often decrease the distribution network total costs and its effect on voltage quality can also be positive, the number of real distribution network implementations is still very low and the distribution network operators (DNOs) do not consider active voltage control as a real option in distribution network planning. Some work is, hence, still needed to enable widespread utilization of active voltage control. This thesis aims at overcoming some of the barriers that are, at present, preventing active voltage control from becoming business as usual for the DNOs. In this thesis, active voltage control methods that can be easily implemented to real distribution networks are developed. The developed methods are, at first, tested using time domain simulations. Operation of one coordinated voltage control (CVC) method is tested also using real time simulations and finally a real distribution network demonstration is conducted. The conducted simulations and demonstrations verify that the developed voltage control methods can be implemented relatively easily and that they are able to keep all network voltages between acceptable limits as long as an adequate amount of controllable resources is available. The developed methods control the substation voltage based on voltages in the whole distribution network and also reactive and real powers of distributed energy resources (DERs) are utilized in some of the developed CVC methods. All types of DERs capable of reactive or real power control can be utilized in the control. The distribution network planning tools and procedures used currently are not capable of taking active voltage control into account. DG interconnection planning is based only on two extreme loading conditions (maximum generation/minimum load and minimum generation/maximum load) and network effects and costs of alternative voltage control methods cannot be compared. In this thesis, the distribution network planning procedure is developed to enable comparison of different voltage control strategies. The statistical distribution network planning method is introduced and its usage is demonstrated in example cases. In statistical distribution network planning, load flow is calculated for every hour of the year using statistical-based hourly load and production curves. When the outputs of hourly load flows (e.g. annual losses, transmission charges and curtailed generation) are combined with investment costs the total costs of alternative voltage control strategies can be compared and the most cost-effective approach can be selected. The example calculations show that the most suitable voltage control strategy varies depending on the network and DG characteristics. The studies of this thesis aim at making the introduction of active voltage control as easy as possible to the DNOs. The developed CVC methods are such that they can be implemented as a part of the existing distribution management systems and they can utilize the already existing data transfer infrastructure of SCADA. The developed planning procedure can be implemented as a part of the existing network information systems. Hence, the currently used network planning and operational tools do not need to be replaced but only enhanced.